Mechanistic Insight into the Synthesis of Higher Alcohols from Syngas

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Letter

Mechanistic insight into the synthesis of higher alcohols from syngas: the role of K promotion on MoS2 catalysts Vera Patrícia Santos, Bart van der Linden, Adam Chojecki , Gerolamo Budroni , Steven L Corthals, Hirokazu Shibata, Garry R. Meima, Freek Kapteijn, Michiel Makkee, and Jorge Gascon ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs4003518 • Publication Date (Web): 18 Jun 2013 Downloaded from http://pubs.acs.org on June 18, 2013

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Mechanistic insight into the synthesis of higher alcohols from syngas: the role of K promotion on MoS2 catalysts Vera P. Santos1, Bart van der Linden1, Adam Chojecki2, Gerolamo Budroni3, Steven Corthals3, Hirokazu Shibata2, Garry R. Meima2,3, Freek Kapteijn1, Michiel Makkee1, and Jorge Gascon1* 1

Catalysis Engineering-Chemical Engineering Dept, Delft University of Technology, Julianalaan 136, 2628 BL Delft 2 Core R&D, Dow Benelux B.V. P.O. Box 48, 4530 AA, Terneuzen (The Netherlands) 3 Hydrocarbons R&D, Dow Benelux B.V. P.O. Box 48, 4530 AA, Terneuzen (The Netherlands) KEYWORDS: Biomass, syngas, higher alcohols, molybdenum disulfide promoter potassium Supporting Information Placeholder ABSTRACT: Operando infrared spectroscopy in combination with a kinetic study are used to elucidate the role of potassium on the conversion of carbon monoxide over K promoted MoS2 catalysts. More specifically, the initial break-in transient has been studied in detail. Stabilization of reaction intermediates, and effect of promoter on the intrinsic properties of MoS2 are discussed. Adsorbed alkoxy species were found to play an important intermediate role in the syngas to alcohol route, and it was found that potassium stabilizes these species. Moreover, the electronic properties of MoS2 change upon promotion, thereby allowing for a relatively easier activation of CO molecule and a reduced hydrogenation activity towards alkanes. In the last few decades, much attention has been paid to higher alcohols synthesis from coal or natural gas via syngas. Higher alcohol synthesis is of interest due to the increasing petroleum prices, environmental concerns, and gasoline additive octane demands. Moreover, higher alcohols are preferred as gasoline boosters over the widely used methanol and ethanol because of their lower volatility, higher solubility in hydrocarbons, and higher calorific value. [1]

Coal, biomass, and natural gas can be converted via FischerTropsch synthesis to higher alcohols over molybdenum, copper, iron, cobalt or tungsten catalysts. [2] MoS2 is one of the most promising catalytic systems, since it shows high resistance to sulphur poisoning and deactivation by coking. [3] In contrast to un-promoted MoS2 that produces mainly hydrocarbons (C1-C6) at typical synthesis conditions (p > 30 bar and T > 250 ºC), alkali promoted MoS2 display a high selectivity towards higher alcohols. [3-4] Alkali promotion tunes kinetics and energetics of the adsorption of the reactants, hydrogen and carbon monoxide, thereby affecting their relative surface coverage during reaction; [5] more specifically, alkali has been claimed to simultaneously be responsible for an increased nondissociative adsorption of CO and to reduce the ability to

activate hydrogen at the MoS2 surface, through site blocking on the sulphide surface. [3] Moreover, some authors also suggest that the addition of potassium stabilizes oxygenate species, directing the synthesis towards alcohols rather than hydrocarbons. [6] However, hardly any experimental evidence has been presented to support these claims under industrially relevant reaction conditions. [3] Here, we studied the role of alkali promotion on CO activation, stabilization of intermediates, and intrinsic properties of MoS2 by combining operando spectroscopy and high-throughput (HT) kinetic experiments. Although several in-situ studies have been reported on alkali promoted supported MoS2 catalysts for the synthesis of higher alcohols, [7] only a few articles deal with infrared spectroscopy studies on bulk molybdenum sulphides. [4a, 4b] Surprisingly in these studies KBr was used as diluent, something striking when the role of potassium is to be unravelled. In this work, we studied a series of bulk MoS2 catalysts synthesized by thermal decomposition of ammonium tetrathiomolybdate. Alkali promotion was performed by physically mixing K2CO3 with MoS2 (see supplementary info for a detailed catalyst preparation). The conversion profile and alcohol selectivity as a function of time on stream (TOS) for all catalysts are shown in Figure 1a. As expected, the presence of potassium on MoS2 shifts the selectivity towards alcohols (C1-C4 alcohols, 45% selectivity) and decreases the break-in period of MoS2 (see inset Figure 1a). Selectivity to alcohols on K-MoS2 increases monotonically with time on stream. This transient period was associated with the spreading of the alkali promoter on the surface of MoS2. [3-4, 6c, 8] The promoter effect is also reflected in the observed apparent activation energy, that decreases from 133 ± 2 to 60 ± 2 kJ/mol upon K promotion (see SI). However, the chain growth probability for hydrocarbons, α, remains constant, as depicted by the unchanged slope in the Anderson–Schultz–Flory (ASF) plots (Figure 1b).

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Figure 1- a) Conversion (X, open symbols) and alcohol selectivity (Salcohols, solid symbols) profiles; b) ASF distribution plot of hydrocarbons and corresponding α values for MoS2 and K-MoS2. Catalytic tests were performed at 310 °C, 90 bar, and space velocity of 10,000 h-1. Selectivity to alcohols was calculated from the individual selectivities of methanol, ethanol, n-propanol and n-butanol. The effect of potassium on the stabilization of surface intermediates and mechanism of alcohol formation was investigated by operando infrared spectroscopy (30 bar, 350 °C and a bed volume space velocity of 7,000 h-1). To validate the operando DRIFTS at lower pressures, additional kinetic experiments were carried out at 30 bar, 350 °C and a bed volume space velocity of 4,000 h-1. An excellent match between the High Throughput (HT) experiments and operando DRIFTS was observed, and the same conversion level was achieved in both experimental setups (X ≈ 2-4%). The conditioning of molybdenum sulphide catalysts is characterized by long transient periods (‘break-in’; Figure 1a), accompanied by a continuous decrease in activity due to the permanent reconstruction of the active phase. However, the major significant differences in the evolution of the conversion of carbon monoxide and in product distribution occur during the first 24 hours. For this reason, the DRIFT operando study was focused on this break-in period. The time resolved DRIFTS spectra recorded during reductive polymerization of CO on K-MoS2 are shown in Figure 2a.

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Figure 2- a) In situ DRIFTS spectra of K-MoS2 during catalytic hydrogenation of CO at 350 °C, 30 bar, CO/H2: 1 and space velocity of 7,000 h-1; b) Evolution of absorbance of hydrocarbons (@ 3010 cm-1) and alcohols (@ 1030 cm-1)as a function of TOS. The formation of hydrocarbons (mainly CH4) and CO2 (gas phase) already from the very beginning of the break-in period is evidenced by the appearance of bands at 3014 and 2360 cm-1, respectively. After approximately 3 h of reaction (induction period) another contribution at 1030 cm-1 appears that is associated with the formation of alcohols (methanol and ethanol). At longer reaction times, bands between 1700-1500 cm-1 (ν COO) also suggest the formation of other oxygenates, like esters. The evolution of hydrocarbons and alcohols as a function of time on stream is shown in Figure 2b. A qualitative reproduction of the break-in profiles obtained from the HTs studies is obtained during the in-situ DRIFTS experiments. The time resolved DRIFTS spectra for the unpromoted system is summarized in Figure S2. To elucidate the role of the alkali dispersion on the induction period, a second promoted sample was prepared by incipient wetness impregnation instead of physical mixing (Figure S3). Using this preparation approach, alcohols were already produced in the first minutes of reaction: clearly the induction period is related to the potassium migration/redistribution and can be significantly shortened. This preparation approach was also used to test the promoter effect of precursors considered in the literature as ‘not selective’ to alcohols, like K2SO4. [4c] In this case the differences in selectivity are even more evident (Figure S3b): catalysts prepared by physical mixing do not produce alcohols, in agreement with literature, [4c]

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while in the case of incipient wetness impregnation, alcohols were formed after 30 minutes of reaction. Although the degree of promotion is lower than that for K2CO3, these findings demonstrate that the nature of the promotional effects does not only depend on the precursor [4c, 6c, 9], but on the way the alkali is incorporated. The infrared spectrum of adsorbed species (after flushing the system with He) on promoted and un-promoted MoS2 after 24 h TOS is presented in Figure 3. In the case of unpromoted sample, the spectrum shows absorption bands in the region of 3120-2860 (ν C-H), 2180-2020 (ν C-O), 16001300 (ν COO) and 1250-1146 cm-1 (ν S-O), assigned to the presence of adsorbed hydrocarbons, CO, oxygenates and oxidised sulphide species, respectively. The vibrational bands at 2920 (ν comb), 2860 (ν C-H), 1590 (νa COO), 1360-1311 (ν s COO), [7a, 10] show that formates are the main oxygenates on the surface of MoS2. The intensity of these bands is low due to deposition of carbon (e.g oligomerization, Boudouard reaction), as demonstrated by Raman spectroscopy (see SI). In the case of the promoted sample (K-MoS2), formate species (1570 (νasCOO), 1401 (ν sCOO), were also found, but the observed frequencies are shifted downward (≈ 20 cm-1), which can be due to a different coordination with Mo surface or interaction with potassium cations. Moreover, besides formates, alkoxy species are also observed on the surface of the promoted sample (2950-2930 (νaCH3), 2860 (ν s CH3), 1135 (ρ C-H) , and 1050 (ν C-O) cm-1). [11] The measured vibrational frequencies suggest that there is an electrostatic interaction between potassium and alkoxy species, forming stabilized complexes. These complexes can also explain the formation of methyl formates on K-MoS2, [6a]

The effect of potassium on the adsorption properties of MoS2 was evaluated by infrared spectroscopy, using CO as a probe molecule.

Figure 3- DRIFTS spectra of MoS2 (down) and KMoS2 (up) after 24 h of reaction followed by flushing with helium at 350 °C and 1 bar (inset represents the C-H stretching region).

Electron probe microscopy analysis performed on the promoted catalyst showed that the distribution of K on the fresh K-MoS2 catalyst is inhomogeneous and that a redispersion of K occurs upon exposure to reaction conditions (Figure S3). Because of the re-dispersion of K during the first reaction hours and the observed induction period for the production of alcohols, CO adsorption studies under IR were carried out after exposing K-MoS2 to syngas for 3 h, at 350 °C and 30 bar. For comparison, the un-promoted sample underwent the same treatment. The ν (CO) stretching region of adsorbed carbon monoxide on these samples is shown in Figure 4. CO interaction with MoS2 after syngas conditions at 350 °C gives rise to absorption bands at 2180, 2100, 2070 and 2042 cm-1. The first band corresponds to the fundamental C-O stretching mode of CO perturbed by the electrostatic field created by cations in Mo4+←CO species. The bands at 2100, 2070, and 2042 cm-1 correspond to the end-on adsorption of CO on coordinatively unsaturated sites (cus) of Moδ+ (δ ≅ 2) having different local environments (edges and corners, respectively). [12] These reduced sites are created upon removal of sulphur at the edge and corner positions, exposing cus of Moδ+. The presence of potassium significantly increases the back donation of Mo, as observed by the shift of the absorption bands to lower wavenumbers. Moreover, the relative amount of edge sites decreased (band at 2100 cm-1), increasing that of corner sites, suggesting that potassium occupies many of these sites, decreasing the hydrogenation ability of promoted MoS2. In addition, the decrease in the absorption band at 2180 cm-1 is indicative of a lower intrinsic Lewis acidity of MoS2. In view of these results, we can rationalize the promotional effect of K as a charge transfer to the catalyst surface due to the low ionization potential of the alkali metal, decreasing both the work function of MoS2 surface and its hydrogenation ability. [5a, 13] In this way, the non-dissociative adsorption of CO is favoured against the hydrogen assisted adsorption path (via formyl species, CHO), that readily would result in the formation of carbon and CHx species on the sulphide surface. [5]

Figure 4- DRIFT spectra of adsorbed CO at room temperature on samples after syngas conversion at 350° C and 30 bar for 3 h.

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Accordingly to obtained spectroscopic and kinetic data and to literature, the activation of CO over un-promoted MoS2 proceeds via a hydrogen assisted process through formyl intermediates (Figure S6). [3, 14] These formyl (HC=O) species are mainly converted into CHx and carbon deposits at the surface of MoS2. The high hydrogenation activity of this catalyst leads mainly to hydrocarbons. [6b, 15] As observed by FT-IR, potassium promotion results in the formation of new active sites for activation of CO and drives the hydrogenation toward partially hydrogenated intermediates (alkoxy). We speculate that methoxides stabilized at the surface can either desorb in the form of methanol or react further with CO giving methyl formates. [6a] In a similar fashion, the presence of potassium in close vicinity to an adsorbed methyl group will favour CO insertion stabilizing acyl intermediates [6c] that are further hydrogenated to ethoxy and finally to ethanol. [6a] For the formation of higher alcohols (C3, C4) on the promoted catalysts, two different pathways would be feasible: (i) CO insertion [6a] and (ii) direct aldol condensation mechanism to a minor extent (Figure S7). [3, 6a, 16] In order to check to what extent aldol condensation plays an important role in the production of higher alcohols, additional kinetic experiments were performed with cofeeding syngas and ethanol. In the case of MoS2, the product stream only contained hydrocarbons (mainly ethane): as expected the conversion of alcohols to hydrocarbons takes place when no alkali is present. In contrast, in case of K-MoS2, the addition of ethanol to the syngas feed boosted the production of higher alcohols, predominantly 1-propanol (dehydroxylation on Mo sites, followed by CO insertion) and 1-butanol (Figure S8). Further, to reveal whether 1-butanol results from the selfcoupling of ethanol or from consecutive CO addition reactions, we performed experiments co-feeding only ethanol and nitrogen. 1-butanol was indeed obtained along with ethyl acetate formed via ethanol dehydrogenation (main product in the absence of hydrogen). [17] From these experiments we conclude that formation of higher alcohols does not only proceed via CO insertion, but also via aldol condensation of shorter alcohols. [18] Altogether, our results emphasize the complexity behind high alcohol synthesis from syngas and the delicate interplay between hydrocarbon and alcohol formation: e.g. the formation of butanol may proceed via: (i) formation of a propyl species on the surface of the catalyst, migration to a K site and CO insertion with stabilization of the corresponding butoxide or (ii) via aldol condensation of ethanol with its corresponding aldehyde. Summarizing, by combining an kinetic investigation and operando spectroscopy the role of K as promoter in MoS2 catalysts was enlightened. The presence of K does not only change the electronic properties of the catalyst, but it also stabilizes alkoxy species, contributing to the formation of higher alcohols via CO insertion and to a small extent by basic catalyzed aldol condensation. Moreover, the promotional effect depends strongly on the way the alkali is incorporated. These insights can contribute to further improvements in the development of selective catalysts for the synthesis of higher alcohols from syngas

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ASSOCIATED CONTENT Supporting Information. Experimental details and additional figures.

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Promoting stability: The role of K as promoter in MoS2 catalysts was enlightenend by combining high-throughput experimentation and operando spectroscopy. The presence of K does not only change the electronic properties of the MoS2 catalyst, but it also stabilizes alkoxy species, contributing to the formation of higher alcohols via CO insertion and to a small extent by basic catalyzed aldol condensation.

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